Ferroelectric, memory device and their manufacturing methods

ABSTRACT

It is intended to provide a ferroelectric that exhibits superior ferroelectricity. A ferroelectric provided is an oxide having a layered crystal structure that is composed of Bi, a first element Me, a second element R, and O. The first element Me is at least one element selected from the group consisting of Na, K, Ca, Ba, Sr, Pb, and Bi. The second element R is at least one element selected from the group consisting of Fe, Ti, Nb, Ta, and W. Ninety-eight percent or more of the entire body of the ferroelectric exhibits ferroelectricity. After an oxide having a layered crystal structure has been grown by a vapor-phase method (crystal growth step), electrodes are attached to the oxide having a layered crystal structure and a voltage is applied thereto (voltage application step). As a result, strains of crystal lattices are corrected at least partially, whereby portions that did not exhibit ferroelectricity at all or did not exhibit superior ferroelectricity due to such large strains that the symmetry of crystal lattices is lost are changed so as to exhibit superior ferroelectricity.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to ferroelectrics such as oxideshaving a layered crystal structure that belong to what is called theaurivillius crystallographic group and that tend to have strains incrystal lattices. The invention also relates to a memory device usingsuch a ferroelectric and manufacturing methods of the ferroelectric andthe memory device.

[0003] 2. Description of the Related Art

[0004] Ferroelectrics have hysteresis in their electric field vs.polarization characteristic. Based on this fact, proposals forrealizing, by utilizing ferroelectrics, nonvolatile memory devices thatcan hold data without being backed up by a power supply were made in1960'. However, the attempts to develop such nonvolatile memory deviceswere stopped because at that time the ferroelectric thin film formingtechnology has not been established yet and there occurred variousproblems such as interference between memory cells. However, because ofmarked improvements in the thin-film technology that occurredthereafter, in recent years attempts to develop nonvolatile memorydevices utilizing ferroelectrics have become active again. (For example,refer to C. Araujo, J. Scott, R. Goddfrey, and L. McMillan, Appl. Phys.Lett., 48 (1986) 1439; and W. Kinney, W. Shepherd, W. Miller, J. Evans,and R. Womack, IEDM Tech. Dig., (1987) 850.)

[0005] Among ferroelectrics to constitute nonvolatile memory devices,bismuth strontium tantalate (Bi₂SrTa₂O₉; hereinafter referred to as“BiSTa”) particularly attracts attention which belongs to what is calledthe aurivillius crystallographic group and is superior in fatiguecharacteristic. (For example, refer to C. A-Paz de Araujo, J. D.Cuchiaro, L. D. McMillan, M. C. Scott, and J. F. Scott, Nature, 374(1995) 627; K. Amanuma, T. Hase, and Y. Miyasaka, Appl. Phys. Lett., 66(1995), 221; and S. B. Desu and D. P. Vijay, Master Sci. and Eng., B32(1995) 75.) The aurivillius crystallographic group includes crystalsthat are represented by a stoichiometric composition formula[Bi₂O₂]²⁺[Me_(m−1)R_(m)O_(3m+1)]²⁻ where m is an integer of 2 or more,Me is at least one element selected from the group consisting of sodium(Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lead(Pb), and bismuth (Bi), R is at least one element selected from thegroup consisting of iron (Fe), niobium (Nb), tantalum (Ta), and tungsten(W).

[0006] Recently, there have been made reports of successes in producinga BiSTa thin film by MOCVD (metal organic chemical vapor deposition).(Refer to T. Ami, K. Hironaka, C. Isobe, N. Nagel, M. Sugiyama, Y.Ikeda, K. Watanabe, A. Machida, K. Miura, and M. Tanaka, Mater. Res.Soc. Symp. Proc., 415 (1996) 195; and T. Li, Y. Zhu, S. B. Desu, C-H.Peng, M. Nagata, Appl. Phys. Lett. 68 (1996) 616.)

[0007] However, examinations of the characteristics of actually producedBiSTa single crystals revealed that one crystal had portions exhibitinganisotropy and portions not exhibiting anisotropy though these portionsdid not have any difference composition. Further, observations with atransmission electron microscope (TEM) showed that portions notexhibiting anisotropy had such large strains that the symmetry ofcrystal lattices was lost. Similar lattice strain was found in BiSTapolycrystalline thin films by using a TEM. That is, it was found thatmaterials having a complex crystal structure such as BiSTa have aproblem that they tend to have such large strains that the symmetry ofcrystal lattices is lost and hence tend to have portions not exhibitinganisotropy and portions exhibiting anisotropy but not showing superiorcharacteristics.

[0008] For the above reasons, when a memory device is formed by using aferroelectric having a complex crystal structure such as BiSTa, itscharacteristics deteriorate depending on the volume ratio of portionsnot exhibiting anisotropy and portions not showing superiorcharacteristics. Further, the device characteristics vary depending onthe proportion of portions not exhibiting anisotropy and portions notshowing superior characteristics.

SUMMARY OF THE INVENTION

[0009] The present invention has been made in view of the aboveproblems, and an object of the invention is therefore to provide aferroelectric that exhibits superior characteristics, a memory deviceusing such a ferroelectric, and their manufacturing methods.

[0010] A ferroelectric according to the invention is such that 98% ormore of the entire body exhibits ferroelectricity.

[0011] In a memory device according to the invention, a pair ofelectrodes are connected to a ferroelectric film, and 98% or more of asection of the ferroelectric film to which a voltage is to be appliedvia the electrodes is a ferroelectric exhibiting ferroelectricity.

[0012] A manufacturing method of a ferroelectric according to theinvention comprises a crystal growth step of growing a crystal that isto constitute the ferroelectric; and a voltage application step ofapplying, after at least part of the crystal has been grown, a voltageto at least part of the crystal to at least partially correct strains ofcrystal lattices existing in the crystal.

[0013] According to the invention, a method for manufacturing a memorydevice in which a pair of electrodes are connected to a ferroelectricfilm, comprises a ferroelectric film forming step of forming aferroelectric film; and a voltage application step of applying, after atleast part of the ferroelectric film has been grown, a voltage to atleast part of the ferroelectric film to at least partially correctstrains of crystal lattices existing in the ferroelectric film.

[0014] In the above ferroelectric, 98% or more of the entire bodyexhibits ferroelectricity, whereby the ferroelectric exhibit superiorcharacteristics.

[0015] In the above memory device, polarization occurs in theferroelectric film when a voltage is applied between the pair ofelectrodes. The voltage vs. polarization characteristic of theferroelectric film has hysteresis. Data storage and readout areperformed by utilizing the hysteresis. Superior characteristics can beobtained because the ferroelectric film is made of a ferroelectric inwhich 98% or more of the section of the ferroelectric film to which avoltage is applied via the electrodes exhibits ferroelectricity.

[0016] In the above manufacturing method of a ferroelectric, after atleast part of a crystal to constitute the ferroelectric is grown in thecrystal growth step, a voltage is applied to at least part of thecrystal in the voltage application step. As a result, at least part ofstrains of crystal lattices existing in the crystal are corrected.

[0017] In the above manufacturing method of a memory device, after atleast part of a ferroelectric film is formed in the ferroelectric filmforming step, a voltage is applied to at least part of the ferroelectricfilm in the voltage application step. As a result, at least part ofstrains of crystal lattices existing in the crystal are corrected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a conceptual diagram showing the crystal structure of anoxide having a layered crystal structure according to a first embodimentof the present invention;

[0019]FIGS. 2A and 2B show an X-ray diffraction pattern that indicatescomposition of crystal-1 and a Rietveld simulation pattern,respectively;

[0020]FIG. 3 is an optical microscope photograph showing the surface ofcrystal-1;

[0021]FIG. 4 is a polarizing microscope photograph taken under thecrossed Nicols condition and showing a light and shade variation ofcrystal-1;

[0022]FIG. 5 is a polarizing microscope photograph taken under thecrossed Nicols condition and showing a light and shade variation ofcrystal-1 in a state that crystal-1 was rotated by 45° from the state ofFIG. 4;

[0023]FIG. 6 is a polarizing microscope photograph taken under thecrossed Nicols condition and showing a light and shade variation ofcrystal-2;

[0024]FIG. 7 shows the configuration of an apparatus used forobservation of a crystal variation in a voltage application step;

[0025] FIGS. 8-10 are polarizing microscope photographs showing avariation in optical anisotropy of crystal-1 in the voltage applicationstep;

[0026]FIG. 11 is a graph showing a ferroelectric hysteresis loop ofcrystal-1;

[0027]FIG. 12 is a graph showing a relationship for crystal-1 betweenthe voltage application time and Pr (spontaneous polarization value);and

[0028]FIG. 13 shows the configuration of a memory cell according to asecond embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The embodiments of the present invention will be hereinafterdescribed in detail with reference to the drawings.

Embodiment 1

[0030] A ferroelectric according to this embodiment is a single crystaloxide having a layered crystal structure that is composed of bismuth, afirst element Me, a second element R, and oxygen. The first element Meis at least one element selected from the group consisting of sodium,potassium, calcium, barium, strontium, lead, and bismuth. The secondelement R is at least one element selected from the group consisting ofiron, titanium, niobium, tantalum, and tungsten. It is preferable thatthe first element Me be at least one element selected from the groupconsisting of strontium, lead, barium, and calcium and that the secondelement R be at least one element selected from the group consisting ofniobium and tantalum. It is most desirable that the first element Me bestrontium and the second element R be tantalum.

[0031] Chemical formula (1) is the stoichiometric composition formula ofthe above oxide having a layered crystal structure. However, theferroelectric of this embodiment is not limited to the one having thestoichiometric composition and includes ones whose compositions aredeviated from the stoichiometric composition.

[Bi₂O₂]²⁺[Me_(m−1)R_(m)O_(3m+1)]²⁻.  (1)

[0032] where

[0033] Me: first element;

[0034] R: second element; and

[0035] m: integer that is one of 2 to 5.

[0036]FIG. 1 shows the crystal structure of an oxide having a layeredcrystal structure in which m is equal to 2 in the composition formula ofchemical formula (1). As shown in FIG. 1, this oxide having a layeredcrystal structure is configured in such a manner that layers 11corresponding to [Bi₂O₂]²⁺ and layers 12 corresponding to [MeR₂O₇]²⁻ arestacked alternately, and has anisotropic c-axis cleavage (refer to H.Maeda, Y. Tanaka, M. Fukutomi, and T. Asano, Jpn. J. Appl. Phys., 27(1988) L209; and K. Hiraga, M. Hirabayashi, M. Kikuchi, and Y. Syono,Jpn. J. Appl. Phys., 27 (1988) L573). This oxide having a layeredcrystal structure also has a feature that the lattice constants of thea-axis and the b-axis are not equal to each other and hence the oxideexhibits ferroelectricity in a plane perpendicular to the c-axis.

[0037] This oxide having a layered crystal structure tends to have suchlarge strains that the symmetry of crystal lattices is lost. Portions inwhich crystal lattices are strained are lowered in ferroelectriccharacteristic depending on the degree of strains, and do not exhibitferroelectricity at all in some cases. However, in this ferroelectric,strains of crystal lattices are corrected at least partially and hence98% or more of the entire body exhibits ferroelectricity.

[0038] The ferroelectric having the above constitution can be producedin the following manner.

[0039] First, a proper oxide material and bismuth oxide (Bi₂O₃) as fluxare mixed with each other. A resulting mixture is put in a propercrucible and the crucible is then placed in a proper heating furnace,where heating and vaporization is effected. For example, the material iscompletely melted by performing first heating for a predetermined timeat a temperature (for instance, 1,350° C.-1,500° C.) higher than orequal to the melting point of the mixture of the material and the flux.Then, the molten material is vaporized by performing second heating fora predetermined time at a temperature (for instance, 1,000° C.-1,300°C.) lower than the melting point. As a result, a single crystal of anoxide having a layered crystal structure is deposited from the vaporphase of the material on a deposition section that is provided at aproper position in the furnace (crystal growth step). The temperature ofthe deposition section is set somewhat (for instance, about 5° C.-20°C.) lower than the material heating temperature.

[0040] Then, after proper electrodes are attached to the thus-depositedsingle crystal oxide having a layered crystal structure, a voltage isapplied to it in the direction perpendicular to the c-axis (voltageapplication step). As a result, strains of crystal lattices in the oxidehaving a layered crystal structure are corrected at least partially.That is, portions that did not exhibit ferroelectricity at all or didnot exhibit superior ferroelectricity due to such large strains that thesymmetry of crystal lattices is lost come to exhibit superiorferroelectricity.

[0041] The voltage applied in the voltage application step may be eitheran AC voltage or a DC voltage. Where a DC voltage is used, pulses ofopposite electric field directions are applied alternately. It ispreferable that the amplitude of the application voltage be such as tocause an electric field that is 1.5 times or more stronger than thecoercive field. A larger application voltage provides a higher effect.During the voltage application, the oxide laving a layered crystalstructure may be heated at a proper temperature or kept at the roomtemperature without heating it (i.e., no temperature adjustment isperformed). However, since the coercive field decreases as thetemperature increases, by heating the oxide a profound effect can beobtained even with a low application voltage. In this manner, aferroelectric according to this embodiment is obtained.

[0042] Because 98% or more of the entire body exhibits ferroelectricity,the ferroelectric of this embodiment can be improved as a whole inferroelectric characteristic. By forming a ferroelectric, nonvolatilememory device by using the ferroelectric of this embodiment, not onlycan the quality of the memory device be improved but also a variation inquality among memory device products can be reduced.

[0043] According to the manufacturing method of a ferroelectric of thisembodiment, because a voltage is applied to a deposited oxide having alayered crystal structure, strains of crystal lattices in the oxidehaving a layered crystal structure can be corrected. Therefore, portionsthat did not exhibit ferroelectricity at all or did not exhibit superiorferroelectricity due to such large strains that the symmetry of crystallattices is lost can be caused to exhibit superior ferroelectricity.Therefore, the ferroelectric of this embodiment can be realized easily.

[0044] More specific versions of the above ferroelectric andmanufacturing method will be described below. That is, the followingdescription will be directed to the case of manufacturing aferroelectric that is an oxide having a layered crystal structure(BiSTa) that is composed of bismuth, strontium (first element Me),tantalum (second element R), and oxygen and has a stoichiometriccomposition represented by chemical formula (2).

[Bi₂O₂]²⁺[SrTa₂O₇]²⁻.  (2)

[0045] First, powders of bismuth oxide, strontium carbonate (SrCO₃), andtantalum oxide (Ta₂O₅) as materials (each material was a highest-qualitychemical produced by Kojundo Chemical Co.) were prepared and mixed witheach other at a mole ratio of 79.0:10.5:10.5. Bismuth oxide was used asflux.

[0046] The above materials was put in a platinum crucible, which was inturn placed in an aluminum crucible. The lidded aluminum crucible wasplaced in a proper heating furnace and the materials were vaporized.Specifically, after the materials were melted by performing firstheating at 1,350° C. for 20 hours, they were vaporized by performingsecond heating at 1,200° C. for 850 hours. Crystals were deposited byusing a top portion of the side wall of the platinum crucible as adeposition section. As a result, a plurality of crystals were depositedon the top portion of the side wall of the platinum crucible (crystalgrowth step).

[0047] To confirm that the crystals obtained were BiSTa havingferroelectricity, the crystals (crystal-1 and crystal-2) were subjectedto the following examinations: (1) identification analysis by X-raydiffraction, (2) surface observation with an optical microscope, (3)observation with a polarizing microscope under the crossed Nicolscondition to check whether the crystal belonged to the orthorhombicsystem, (4) chemical composition analysis by EPMA, and (5) observationof ferroelectric hysteresis. An X-ray diffraction apparatus RigakuRAD-IIIB was used for the X-ray diffraction of item (1). As for theEPMA, a wavelength dispersive X-ray spectroscopy (WDS) analysis wasconducted by using CAMEBAXSX-50. Results of the above examinations willbe described below.

[0048] (1) Results of X-ray Diffraction Analysis:

[0049]FIGS. 2A and 2B show an X-ray diffraction pattern (XRDP) ofcrystal-1 and a Rietveld simulation pattern, respectively. This Rietveldsimulation pattern is a reference pattern of BiSTa calculated byRietveld simulation based on the lattice constants that were obtained byRae et al. (a=0.553065 nm, b=0.553445 nm, and c=2.49839 nm; A. D. Rae,J. G. Thompson, and R. L. Withers, Acta. Cryst., B48 (1992) 418.) Indoing the Rietveld simulation, the space group “Fmmm” was used ratherthan “A2₁am” that was the assertion of Rae et al.

[0050] As seen from FIGS. 2A and 2B, the diffraction peaks of the XRDPof crystal-1 coincide with those of the Rietveld simulation pattern andhence it is confirmed that crystal-1 is BiSTa. Further, based on thefact that diffraction peaks (006), (0010), etc. of the XRDP of crystal-1are higher than those of the Rietveld simulation pattern, it is foundthat crystal-1 has strong c-axis orientation due to its flake-likeshape. Results similar to those with crystal-1 were obtained withcrystal-2, though no figure is presented here.

[0051] (2) Results of Surface Observation with Optical Microscope:

[0052]FIG. 3 is an optical microscope photograph of crystal-1. From FIG.3, it is found that crystal-1 has a smooth surface. Since BiSTa hasc-axis cleavage as understood from the crystal structure shown in FIG.1, this smooth surface is considered to be the c-plane. Results similarto those with crystal-1 were obtained with crystal-2, though no figureis presented here.

[0053] (3) Results of Observation with Polarizing Microscope UnderCrossed Nicols Condition:

[0054] The crystal obtained was mounted on a rotary stage that isdisposed between a pair of polarizers in the crossed Nicols condition.While the stage was rotated for the c-plane, it was checked whether alight and shade pattern occurred in synchronism with the rotation. Aperiodic light and shade response was observed with each of crystal-1and crystal-2. FIGS. 4 and 5 are polarizing microscope photographsbefore and after crystal-1 was rotated by 45°. As seen from thesefigures, it is confirmed that the lattice constants of the a-axis andthe b-axis are different from each other in both crystal-1 and crystal-2(no figures are presented for crystal-2). Further, as seen from FIG. 5,crystal-1 had portions exhibiting a periodic light and shade responseand portions not exhibiting such a response. This was also the case withcrystal-2. In FIG. 5, the portions exhibiting a light and shade responseare not clearly distinguished from the portions not exhibiting such aresponse. FIG. 6 is a polarizing microscope photograph of crystal-2 inwhich the two kinds of portions are clearly distinguished from eachother.

[0055] (4) Results of Chemical Composition Analysis by EPMA

[0056] Table 1 shows ratios in composition among bismuth, strontium, andtantalum. Although the composition of each of crystal-1 and crystal-2 isclose to the stoichiometric composition as seen from Table 1, it wasfound that the composition of each of crystal-1 and crystal-2 wasdeviated from the stoichiometric composition slightly in such a degreethat no difference was found in the X-ray diffraction patterns of item(1). It was also found that there was no difference in compositionbetween portions exhibiting a light and shade response in the polarizingmicroscope observation under the crossed Nicols condition and portionsnot exhibiting such a response (see FIGS. 5 and 6) of item (4).

Table 1

[0057] Crystal-1: Bi:Sr:Ta=2.07:1.10:2.00

[0058] Crystal-2: Bi:Sr:Ta=2.10:1.01:2.00

[0059] (5) Results of Observation of Ferroelectric Hysteresis:

[0060] Gold (Au) electrodes were evaporated on one surface (c-plane) ofcrystal-1 at an interval of 500 μm, and ferroelectric hysteresis wasobserved while a voltage of 820 V was applied in the directionperpendicular to the c-axis and the temperature was kept at 250° C.Further, gold electrodes were evaporated on one surface (c-plane) ofcrystal-2 at an interval of 100 μm, and ferroelectric hysteresis wasobserved while a voltage of 500 V was applied in the directionperpendicular to the c-axis and the temperature was kept at 200° C. Aferroelectric hysteresis loop was observed with both crystal-1 andcrystal-2.

[0061] It was confirmed from the above results that both crystal-1 andcrystal-2 are BiSTa single crystals having ferroelectricity. It was alsofound that each of crystal-1 and crystal-2 is uniform in terms ofcomposition but has portions not exhibiting anisotropy (i.e.,ferroelectricity). In view of this, crystals obtained were subjected toobservation with a transmission electron microscope (TEM), whereby itwas found that portions having disordered (strained) crystal latticesexisted in orderly crystal lattices. The portions having disorderedcrystal lattices corresponded to the portions not exhibitingferroelectricity.

[0062] Then, while a voltage was applied to each of crystal-1 andcrystal-2 (voltage application step), it was observed how the crystalvaried.

[0063]FIG. 7 shows the configuration of an apparatus that was used forthis observation. This apparatus is provided with a polarizingmicroscope 21 in which a pair of polarizers 21 b are disposed in thecrossed Nicols state so that a crystal M that is mounted on a rotarystate 21 a is interposed in between. The polarizing microscope 21 isconnected to a voltage application device 22 for applying a voltage tothe crystal M mounted on the rotary stage 21 a. The voltage applicationdevice 22 is connected to a computer 23, to enable analysis of theelectric field vs. polarization characteristic of the crystal M. A CCDcamera 24 is disposed above the polarizing microscope 21 so that avariation of the crystal M during voltage application can be displayedon a screen 25 in an enlarged manner as well as output from a colorvideo printer 26. The rotary stage 21 a of the polarizing microscope 21is connected to a temperature adjustment device 27, to enable adjustmentof the temperature of the crystal M.

[0064] By using the above-configured apparatus, first, a DC voltage ofabout 820 V (corresponding to an electric field of about 16 kV/cm) wasapplied to crystal-1 that was mounted on the rotary stage 21 a in thedirection perpendicular to the c-axis while heating was made by thetemperature adjustment device 27 to set the temperature at 200° C. Then,a DC voltage of about 820 V was applied in such a manner that theelectric field direction is reversed. At this time, the magnitude of theelectric field was about 1.7 times larger than the coercive field. FIGS.8 and 9 are polarizing microscope photographs of crystal-1 before andafter voltage application, respectively. As seen from FIGS. 8 and 9,portions that did not exhibit, before the voltage application, aperiodic light and shade response when the crystal was rotated under thecrossed Nicols condition were partially changed by the voltageapplication so as to exhibit a light and shade response.

[0065] Then, a variation of crystal-1 was observed with the polarizingmicroscope 21 and the CCD camera 24 while an AC voltage of 60 Hz andabout 820 V (corresponding to an electric field of about 16 kV/cm) wasapplied in the direction perpendicular to the c-axis and heating wasmade to set the temperature at 200° C. FIG. 10 is a polarizingmicroscope photograph of crystal-1 taken after the AC voltage wasapplied for 120 minutes. In this manner, the voltage application causedthe portions that did not exhibit a periodic light and shade responsewhen the crystal was rotated under the crossed Nicols condition to begradually extended perpendicularly and thinned to the voltageapplication direction, and their areas to gradually decrease. Whereasbefore the voltage application the portions that exhibited a light andshade response accounted for about 80% of the entire body (see FIG. 8),after the application of the AC voltage for about 120 minutes suchportions accounted for more than about 99% of the entire body.

[0066] Further, while the variation of crystal-1 due to the voltageapplication was observed with the polarizing microscope 21, the electricfield vs. polarization characteristic of crystal-1 was analyzed at timeswith the computer 23. FIG. 11 shows a ferroelectric hysteresis loop ofcrystal-1 that was obtained after the AC voltage was applied for 100minutes. FIG. 12 shows a variation of Pr (spontaneous polarizationvalue) due to the application of the AC voltage. As seen from FIG. 12,it became apparent that the application of a voltage improves theferroelectricity.

[0067] Then, a DC voltage of about 500 V (corresponding to an electricfield of about 50 kV/cm) was applied to crystal-2 that was mounted onthe rotary stage 21 a in the direction perpendicular to the c-axis whileheating was made by the temperature adjustment device 27 to set thetemperature at 200° C. Thereafter, a DC voltage of about 500 V wasapplied in such a manner that the electric field direction is reversed.At this time, the magnitude of the electric field was about 5.2 timeslarger than the coercive field. As a result, portions that did notexhibit, before the voltage application, a periodic light and shaderesponse when the crystal was rotated under the crossed Nicols conditionwere partially changed by the voltage application so as to exhibit alight and shade response.

[0068] Then, a variation of crystal-2 was observed with the polarizingmicroscope 21 and the CCD camera 24 while an AC voltage of 60 Hz andabout 500 V (corresponding to an electric field of about 50 kV/cm) wasapplied in the direction perpendicular to the c-axis and heating wasmade to set the temperature at 200° C. The voltage application causedthe portions that did not exhibit a periodic light and shade responsewhen the crystal was rotated under the crossed Nicols condition to begradually extended perpendicularly and thinned to the voltageapplication direction, and their areas to gradually decrease. Whereasbefore the voltage application the portions that exhibited a light andshade response accounted for about 85% of the entire body, after theapplication of the AC voltage for about 120 minutes such portionsaccounted for more than about 99% of the entire body. Also as for theferroelectric hysteresis loop of crystal-2, results similar to thosewith crystal-1 were obtained.

[0069] It was confirmed from the above results that the voltageapplication changes portions that did not exhibit ferroelectricity sothat they exhibit ferroelectricity and hence the ferroelectriccharacteristic is improved as a whole.

Embodiment 2

[0070]FIG. 13 shows the configuration of a memory cell according to thisembodiment, which is composed of a switching transistor 30 and a memorydevice 40. The transistor 30 is a MOS (metal-oxide-semiconductor)transistor in which a p-well layer 31 doped with an impurity such asboron (B) is formed on a semiconductor substrate (for instance, ann-type silicon (Si) semiconductor substrate) 51. A source electrode 32that is an n⁺layer doped with an impurity such as phosphorus (P) isformed in a source electrode forming region of the p-well layer 31, anda drain electrode 33 that is also an n⁺ layer is formed in a drainelectrode forming region of the p-well layer 31. A proper gap is formedbetween the source electrode 32 and the drain electrode 33, and a gateelectrode 35 made of polysilicon, for instance, is formed above the gapwith a gate insulating film 34 of silicon dioxide (SiO₂) interposed inbetween.

[0071] As for the memory device 40, a bottom electrode 42 made of aproper metal such as aluminum (Al) is formed on a memory device formingregion of the semiconductor substrate 51 with an interlayer insulatingfilm 41 of silicon dioxide interposed in between. A ferroelectric film43 that is a ferroelectric such as an oxide having a layered crystalstructure that is composed of bismuth, strontium, tantalum, and oxygenis formed on part of the bottom electrode 42. A top electrode 44 made ofa proper metal such as aluminum is formed on the ferroelectric film 43.That is, in the memory device 40, a pair of electrodes, i.e., the bottomelectrode 42 and the top electrode 44, are connected to theferroelectric film 43.

[0072] The ferroelectric constituting the ferroelectric film 43 is suchthat 98% or more of its section to which a voltage is to be applied viathe top electrode 44 and the bottom electrode 42 exhibitferroelectricity. Where the ferroelectric film 43 is made of an oxidehaving a layered crystal structure that is composed of bismuth,strontium, tantalum, and oxygen, it may be either a single crystal or apolycrystal. However, the c-axis should be set perpendicular to thevoltage application direction.

[0073] An interlayer insulating film 52 made of silicon dioxide isformed on the transistor 30 and the memory device 40. A contact hole 52a for contact to the drain electrode 33, a contact hole 52 b for contactto the top electrode 44, and a contact hole 52 c for contact to thebottom electrode 42 are formed through the interlayer insulating film52.

[0074] A pickup electrode 53 made of polysilicon, for instance, isformed on a portion of the drain electrode 33 exposed by the contacthole 52 a. An interconnection 54 made of a proper metal such as aluminumis formed on the pickup electrode 53 and a portion of the top electrode44 exposed by the contact hole 52 b so as to electrically connect thetop electrode 44 and the pickup electrode 53 (i.e., the drain electrode33). Further, an interconnection 55 made of a proper metal such asaluminum is formed on a portion of the bottom electrode 42 exposed bythe contact hole 52 c so as to electrically connect the bottom electrode42 to another device (not shown).

[0075] Although not shown in FIG. 13, contact holes for contact to thesource electrode 32 and the gate electrode 35, respectively, are formedthrough the interlayer insulating film 52. Given interconnections areconnected to the source electrode 32 and the gate electrode 35 via thecontact holes, respectively. When a voltage is applied to the gateelectrode 35, current flows between the source electrode 32 and thedrain electrode 33.

[0076] For example, the memory cell having the above configuration canbe manufactured in the following manner.

[0077] First, a p-well layer 31 is formed on a semiconductor substrate51 by implanting an impurity such as boron. Then, n⁺ layers as thesource electrode 32 and the drain electrode 33 are formed by implantingan impurity such as phosphorus selectively, i.e., into a sourceelectrode forming region and a drain electrode forming region.Subsequently, a gate oxide film 34 is formed by oxidizing the surface ofthe p-well layer 31 in which the source electrode 32 and the drainelectrode 33 have been formed. Thereafter, a gate electrode 35 is formedby laying a polysilicon film on the gate oxide film 34 selectively,i.e., between the source electrode 32 and the drain electrode 33, by CVD(chemical vapor deposition). A transistor 30 is thus formed.

[0078] After the formation of the transistor 30, an interlayerinsulating film 41 is formed on its surface. Then, a bottom electrode 42is formed by evaporating a metal film of aluminum, for instance,selectively, i.e., in a memory device forming region. Subsequently, aferroelectric film 43 is formed by laying a thin film of ferroelectricselectively, i.e., on part of the bottom electrode 42 (ferroelectricfilm forming step). Where the ferroelectric film 43 is an oxide having alayered crystal structure that is composed of bismuth, strontium,tantalum, and oxygen, a crystal of the oxide having a layered crystalstructure may be deposited by a vapor-phase method or may be formed byMOCVD, MOD (metal organic decomposition), a sol-gel method, sputtering,or MBE (molecular beam epitaxy).

[0079] After the ferroelectric film 43 has been formed in the abovemanner, a top electrode 44 is formed by evaporating a metal film ofaluminum, for instance, selectively on the ferroelectric film 43.Thereafter, a voltage is applied to the ferroelectric film 43 via thetop electrode 44 and the bottom electrode 42 (voltage application step).As a result, strains of crystal lattices existing in the voltageapplication section of the ferroelectric film 43 are corrected at leastpartially. That is, portions that did not exhibit ferroelectricity atall or did not exhibit superior ferroelectricity due to such largestrains that the symmetry of crystal lattices is lost come to exhibitsuperior ferroelectricity.

[0080] The voltage applied in the voltage application step may be eitheran AC voltage or a DC voltage. Where a DC voltage is used, pulses tocause opposite electric field directions are applied alternately. It ispreferable that the magnitude of the application voltage be such as tocause an electric field 1.5 times or more stronger than the coercivefield or larger than the memory drive voltage. A larger applicationvoltage can provide a higher effect. During the voltage application, theferroelectric film 43 may be heated properly or kept at the roomtemperature without heating it (i.e., no temperature adjustment isperformed). During the voltage application, the coercive field decreasesas the temperature increases. Therefore, by increasing the applicationvoltage, a profound effect can be obtained even with a low voltage.However, since a high temperature may adversely affect other devices, itis preferable that the temperature not be set too high. The memorydevice 40 is formed in the above manner.

[0081] After the formation of the memory device 40, an interlayerinsulating film 52 is formed on the memory device 40 and the transistor30 and respective contact holes 52 a-52 c for exposing parts of thesurfaces of the drain electrode 33, the top electrode 44, and the bottomelectrode 42 and respective contact holes (not shown) for exposing partsof the surfaces of the source electrode 32 and the gate electrode 35 areformed. Then, a pickup electrode 53 is formed by burying a polysiliconlayer selectively, that is, in the contact hole 52 a by CVD, forinstance. Subsequently, interconnections 54, 55, etc. are formed byevaporating a metal film of aluminum, for instance. As a result, thetransistor 30 and the memory device 40 are electrically connected toeach other, whereby a memory cell is completed as shown in FIG. 13.

[0082] The memory cell that is manufactured as described above operatesin the following manner.

[0083] In the memory cell, when a voltage is applied to the gateelectrode 35 of the transistor 30, the transistor 30 as a switch isturned on and current flows between the source electrode 32 and thedrain electrode 33. As a result, current flows into the memory device 40via the pickup electrode 53 and the interconnection 54 and a voltage isapplied between the top electrode 44 and the bottom electrode 42. Uponthe application of the voltage to the memory device 40, polarizationoccurs in the ferroelectric film 43. The voltage vs. polarizationcharacteristic of the ferroelectric film 43 has hysteresis. Storage orreadout of data “1” or “0” is performed by utilizing the hysteresis.Being made of a ferroelectric in which 98% or more of the voltageapplication section exhibits ferroelectricity, the ferroelectric film 43exhibits superior characteristics. Therefore, the data storage orreadout can be performed with high accuracy.

[0084] In the memory device of this embodiment, since the ferroelectricfilm 43 is made of a ferroelectric in which 98% or more of the voltageapplication section exhibits ferroelectricity, the characteristics canbe improved and a variation among devices can be reduced.

[0085] In the manufacturing of the memory device of this embodiment, theapplication of a voltage to the ferroelectric film 43 can correctstrains of crystal lattices existing in the ferroelectric film 43. As aresult, portions that did not exhibit ferroelectricity at all or did notexhibit superior ferroelectricity due to such large strains that thesymmetry of crystal lattices is lost can be changed so as to exhibitsuperior ferroelectricity. Therefore, the memory device of thisembodiment can be realized easily.

[0086] Although the invention has been described above in the form ofembodiments, the invention is not limited to the embodiments and variousmodifications are possible within the range of equivalence. For example,although the first embodiment is directed to the ferroelectric that isthe oxide having a layered crystal structure composed of bismuth, thefirst element Me, the second element, and oxygen (the first element Meis at least one element selected from the group consisting of sodium,potassium, calcium, barium, strontium, lead, and bismuth, and the secondelement R is at least one element selected from the group consisting ofiron, titanium, niobium, tantalum, and tungsten), other ferroelectricscan also be used in the invention.

[0087] Although in the first embodiment the specific embodiment wasdescribed which was directed to the ferroelectric of BiSTa, similarresults can be obtained with other oxides having a layered crystalstructure that are composed of bismuth, the first element Me, the secondelement, and oxygen (the first element Me is at least one elementselected from the group consisting of sodium, potassium, calcium,barium, strontium, lead, and bismuth, and the second element R is atleast one element selected from the group consisting of iron, titanium,niobium, tantalum, and tungsten).

[0088] Further, although the first embodiment is directed to the casewhere after the crystal growth step a voltage is applied to a crystalthat has grown, the invention encompasses a case where a voltage isapplied after at least part of a crystal to constitute a ferroelectrichas grown. The invention also encompasses a case of applying a voltageto part of a crystal that has grown in addition to the case of applyinga voltage to the entire crystal.

[0089] Further, although the second embodiment is directed to the casewhere a voltage is applied to the ferroelectric film 43 after the bottomelectrode 42, the ferroelectric film 43, and the top electrode 44 havebeen formed, a voltage may be applied at any time after at least part ofthe ferroelectric film 43 has been formed.

[0090] In addition, although the second embodiment is directed to thememory cell that is composed of the transistor 30 and the memory device40, the invention is broadly applied to memory devices in which a pairof electrodes are connected to a ferroelectric. Therefore, thetransistor 30 is not limited to the MOS transistor and may be the MESFET(metal semiconductor field-effect transistor) and other kinds oftransistors.

[0091] Further, although the second embodiment is directed to a singlememory cell, the invention can similarly be applied to an LSI (largescale integrated circuit) memory in which a plurality of memory cellsare integrated.

[0092] As described above, according to the ferroelectric of theinvention, the ferroelectric characteristic can be improved as a wholebecause 98% or more of the entire body exhibits ferroelectricity.Therefore, by forming a ferroelectric, nonvolatile memory device byusing this ferroelectric, advantages are obtained that the quality ofthe memory device can be improved and a variation in quality amongmemory device products can be reduced.

[0093] According to the memory device of the invention, since 98% ormore of the section of the ferroelectric film to which a voltage isapplied is a ferroelectric that exhibits ferroelectricity, advantagesare provided that the characteristics can be improved and a variationamong memory devices can be reduced.

[0094] Further, according to the ferroelectric manufacturing method ofthe invention, since after at least part of a crystal to constitute theferroelectric is grown a voltage is applied to at least part of thecrystal, strains of crystal lattices existing in the crystal can becorrected. Therefore, portions that did not exhibit ferroelectricity atall or did not exhibit superior ferroelectricity due to such largestrains that the symmetry of crystal lattices is lost can be caused toexhibit superior ferroelectricity. Therefore, advantages are obtainedthat the characteristics can be improved and the ferroelectric of theinvention can be realized easily.

[0095] In addition, according to the memory device manufacturing methodof the inventions since after at least part of a ferroelectric film isformed a voltage is applied to at least part of the ferroelectric film,strains of crystal lattices existing in the ferroelectric film can becorrected. Therefore, advantages are obtained that the characteristicscan be improved and the memory device of the invention can be realizedeasily.

What is claimed is:
 1. A ferroelectric wherein 98% or more of an entirebody thereof exhibits ferroelectricity.
 2. The ferroelectric accordingto claim 1, wherein the ferroelectric is an oxide having a layeredcrystal structure that is composed of bismuth, a first element, a secondelement, and oxygen, where the first element is at least one elementselected from the group consisting of sodium, potassium, calcium,barium, strontium, lead, and bismuth and the second element is at leastone element selected from the group consisting of iron, titanium,niobium, tantalum, and tungsten.
 3. The ferroelectric according to claim2, wherein the ferroelectric is a single crystal.
 4. The ferroelectricaccording to claim 2, wherein the first element is strontium and thesecond element is tantalum.
 5. A memory device in which a pair ofelectrodes are connected to a ferroelectric film, wherein 98% or more ofa section of the ferroelectric film to which a voltage is to be appliedvia the electrodes is a ferroelectric exhibiting ferroelectricity. 6.The memory device according to claim 5, wherein at least part of theferroelectric film is an oxide having a layered crystal structure thatis composed of bismuth, a first element, a second element, and oxygen,where the first element is at least one element selected from the groupconsisting of sodium, potassium, calcium, barium, strontium, lead, andbismuth and the second element is at least one element selected from thegroup consisting of iron, titanium, niobium, tantalum, and tungsten. 7.The memory device according to claim 6, wherein the first element isstrontium and the second element is tantalum.
 8. A manufacturing methodof a ferroelectric, comprising: a crystal growth step of growing acrystal that is to constitute the ferroelectric; and a voltageapplication step of applying, after at least part of the crystal hasbeen grown, a voltage to at least part of the crystal to at leastpartially correct strains of crystal lattices existing in the crystal.9. The manufacturing method according to claim 8, wherein an AC voltageis applied in the voltage application step.
 10. The manufacturing methodaccording to claim 8, wherein in the voltage application step at leastone pair of DC voltage pulses to cause opposite electric fielddirections are applied alternately.
 11. The manufacturing methodaccording to claim 8, wherein a voltage having such a magnitude as tocause an electric field that is 1.5 times or more stronger than acoercive field is applied in the voltage application step.
 12. Themanufacturing method according to claim 8, wherein the voltageapplication step is executed while heating is performed.
 13. Themanufacturing method according to claim 8, wherein the voltageapplication step is executed while no temperature adjustment is made.14. The manufacturing method according to claim 8, wherein theferroelectric is an oxide having a layered crystal structure that iscomposed of bismuth, a first element, a second element, and oxygen,where the first element is at least one element selected from the groupconsisting of sodium, potassium, calcium, barium, strontium, lead, andbismuth and the second element is at least one element selected from thegroup consisting of iron, titanium, niobium, tantalum, and tungsten. 15.The manufacturing method according to claim 14, wherein the firstelement is strontium and the second element is tantalum.
 16. A methodfor manufacturing a memory device in which a pair of electrodes areconnected to a ferroelectric film, comprising: a ferroelectric filmforming step of forming a ferroelectric film; and a voltage applicationstep of applying, after at least part of the ferroelectric film has beengrown, a voltage to at least part of the ferroelectric film to at leastpartially correct strains of crystal lattices existing in theferroelectric film.
 17. The method according to claim 16, wherein avoltage higher than or equal to a memory drive voltage is applied in thevoltage application step.
 18. The method according to claim 16, whereina voltage having such a magnitude as to cause an electric field that is1.5 times or more stronger than a coercive field is applied in thevoltage application step.
 19. The method according to claim 16, whereinthe ferroelectric is an oxide having a layered crystal structure that iscomposed of bismuth, a first element, a second element, and oxygen,where the first element is at least one element selected from the groupconsisting of sodium, potassium, calcium, barium, strontium, lead, andbismuth and the second element is at least one element selected from thegroup consisting of iron, titanium, niobium, tantalum, and tungsten. 20.The method according to claim 16, wherein the first element is strontiumand the second element is tantalum.